Breastfeeding may protect older mothers from cancer

Women who delay pregnancy past their mid-twenties may reduce their risk of breast cancer later in life by breastfeeding their children, a new study suggests.

Previous research has shown that women who have their first child in their thirties have an increased risk of breast cancer, regardless of how many children they go on to have. But the new study found this link only among older mums who did not breastfeed.

Giske Ursin at Keck School of Medicine in Los Angeles, California, US, and colleagues asked nearly 1000 women between the ages of 55 and 64 who had been diagnosed with breast cancer to provide information about their childbearing and breastfeeding history. The researchers also obtained these details from 1500 healthy women in the same age range.

After comparing the information from the women with breast cancer and their healthy counterparts, Ursin's team found that delaying pregnancy past 25 years of age was associated with an increased risk of breast cancer - but only if the women had not breastfed.

Million-dollar question

Previous studies show having children offers some protection against breast cancer - childless women have a higher risk of the cancer than those who have children. Nevertheless, Ursin's team found that women who gave birth to three children after age 25 and never breastfed had twice the risk of breast cancer as women who had never had children.

Exactly how breastfeeding has a protective effect on breast cancer remains largely unclear. "That's the multi-million dollar question. We do not understand why," says Ursin.

In terms of the general protective effect of childbirth, some believe that hormone changes that women undergo during and after pregnancy change the breast composition in a way that reduces the chance of hormone-sensitive cancers in that tissue.

Breastfeeding may offer additional protection "because the ovary production of hormones is suppressed during breastfeeding, thereby reducing women's exposure to oestrogens and progesterone", suggests Anne McTiernan, director of the prevention centre at the Fred Hutchinson Cancer Research Center in Seattle, Washington, US.

Better out?

Ursin's team also found a link between breastfeeding and a reduced risk of hormone-insensitive breast cancers. "They're definitely harder to treat, so they often have a poorer prognosis," she explains. She says one controversial theory is that breastfeeding somehow "squeezes out carcinogens" that have been building up in the breast. "Milk secretion may mechanically move carcinogens out of the breast ducts," agrees McTiernan.

However, Tim Byers, deputy director of the University of Colorado Cancer Center in Aurora, Colorado, US, believes this is unlikely. "I wouldn't characterise it as total nonsense, but I would put it pretty low on the list of plausible explanations," he told New Scientist.

Ursin presented the findings at the 2007 meeting of the American Association for Cancer Research in Los Angeles.

If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options available for use of articles and graphics we own the copyright to.